1. Field of the Invention
The present invention relates to a transmission device and a transmission/reception apparatus using the transmission device. In particular, the present invention relates to a signal transmission technique using a current as the signal transmission medium.
2. Description of the Related Art
In general, electronic equipment comprises multiple circuits such as a central processing unit, a semiconductor integrated circuit, etc. For example, a cellular phone comprises a communication circuit, a display, an image capturing device, and so on. In such electronic equipment, each circuit executes the processing that corresponds to its function. For example, a communication circuit executes communication processing. A display displays given information. An image capturing circuit executes image capturing processing. Furthermore, such a circuit executes signal transmission processing with respect to another circuit. For example, the image capturing device transmits image data thus captured to a communication circuit. With conventional techniques, voltage is employed as the signal transmission medium between such circuits, and the voltage changes in a range between the power supply voltage and the ground. In recent years, the operation speed of each circuit has been becoming faster. Furthermore, the number of signals to be processed by the central processing device has been becoming larger. This leads to a demand for high speed transmission processing between the circuits. However, voltage signal transmission techniques have the following problem.
In general, differential signal lines between the circuits have capacitance. With such an arrangement, the charging/discharging of the capacitance according to the change in voltage occurs. In this case, the charge amount corresponds to the capacitance. Such an arrangement in which signals are transmitted via voltage requires an additional period of time for storing and releasing the charge that corresponds to the capacitance. This leads to an increased rising period of time and an increased falling period of time, resulting in a problem of difficulty in transmitting signals at a high speed. In order to solve this problem, a signal transmission technique has been proposed, which transmits signals via current instead of voltage (see Patent documents 1 and 2, for example).
[Patent Document 1]
Japanese Patent Application Laid-open No. 2005-64589
[Patent Document 2]
Japanese Patent Application Laid-open No. 2005-64590